Image pick-up device on board a space craft, space craft and image sensing method comprising same

ABSTRACT

The present invention provides a camera device for mounting on board a spacecraft, the camera device having at least one observation sensor and displacement means ( 4, 5, 6, 7, 8, 9 ) for controlled displacement of said device relative to the spacecraft with at least one degree of freedom, the device being characterized in that said displacement means comprise at least one transducer of an active material suitable for deforming dynamically under the effect of a variable electric and/or magnetic field, together with driver means suitable for controlling the transducer so as to displace the observation sensor in compliance with a variable control relationship adapted to compensating movements induced by the flight dynamics of the satellite.

[0001] The invention relates to the field of observation and imagingfrom a spacecraft. More particularly, in one aspect, the inventionprovides a camera device on board such a spacecraft. For example, thecamera device may include a charge-coupled device (CCD) sensor matrix.Such CCD sensors are also referred to below as “photosensitive chargetransfer elements”.

[0002] More particularly, the invention relates to such camera devicesincluding means for compensating smearing due to movements of thespacecraft relative to the object to be observed and/or whose image isto be conserved.

[0003] For example, the invention can be used for imaging the groundfrom a satellite. Under such circumstances, in order to obtain goodresolution, it is necessary to ensure that sufficient energy reacheseach sensor during the time the satellite travels a distancecorresponding to the desired resolution on the ground.

[0004] Known sensor matrices that are available on the market presentradiometric performance requiring an exposure time of about 500microseconds (μs) for a signal-to-noise ratio of 40 decibels (dB). For asatellite traveling at 8 kilometers per second (km/s) with groundresolution R_(s) equal to 1 meter (m), then exposure time cannot exceed⅛^(th) of a millisecond (ms), i.e. 120 microseconds (μs). This exposuretime should even be shorter if it is desired maintain good imagesharpness.

[0005] The quantity of light picked up by each sensor during such anexposure time is therefore insufficient.

[0006] To obtain the necessary quantities of light, telescopes have beenused with apertures that are 3 to 4 times that which is required bydiffraction conditions, so as to pick 10 times more energy. However suchtelescopes are heavy and bulky (with their weight increasing with thesquare of their aperture).

[0007] Another way of obtaining a sufficient quantity of light is tolengthen exposure time and to compensate smearing during the exposuretime by synchronizing displacement of the camera device relative to thesatellite as a function of the motion of the satellite along itstrajectory. This compensation of smearing is sometimes achieved bymodifying light paths, by moving a mirror, a lens, etc.

[0008] Document U.S. Pat. No. 5,460,341 describes a camera device foruse on board a satellite or other spacecraft, the camera deviceincluding a smearing compensation system to correct displacements alongan optical axis, focusing displacements of optical parts, the positionof the camera device itself relative to the craft on which it is fixed,etc. That compensation device includes linear actuators for adjustingthe position of the camera device relative to the spacecraft bycontrolled displacement in several degrees of freedom. The actuatorcomprises a motor having a moving coil or a moving magnet. Those linearactuators are of the contactless type, i.e. there is no friction contactbetween the moving parts relative to one another. Unfortunately, suchdevices become unusable when it is desired to obtain very highresolution in the compensation movement. Under such circumstances,extremely precise displacements and very short response times arerequired, but cannot be obtained with those prior art camera devices.

[0009] An object of the invention is to provide camera devices with adisplacement mechanism that is more precise and that has a response timethat is very short.

[0010] According to the invention, this object is achieved by a cameradevice for mounting on board a spacecraft, the camera device having atleast one observation sensor and displacement means (4, 5, 6, 7, 8, 9)for controlled displacement of said device relative to the spacecraftwith at least one degree of freedom, the device being characterized inthat said displacement means comprise at least one transducer of anactive material suitable for deforming dynamically under the effect of avariable electric and/or magnetic field, together with driver meanssuitable for controlling the transducer so as to displace theobservation sensor in compliance with a variable control relationshipadapted to compensating movements induced by the flight dynamics of thesatellite.

[0011] This type of material makes it possible to obtain displacement ofthe entire camera device relative to the spacecraft with positioningthat is rigid, with high precision, and practically without any timedelay and without using mechanical structures proper.

[0012] The use of an active material suitable for deforming is under theeffect of an electric field and/or a magnetic field, typically apiezoelectric material or a magnetostrictive material, can be envisagedonly for displacements that are very small. However, the opticalresolutions that were accessible with prior art devices were such as torequire compensation displacements of an amplitude that was too greatfor it to have been possible to envisage using such materials.

[0013] The camera device of the invention may present the followingadvantageous but optional characteristics taken separately or incombination:

[0014] the displacement means comprise a plurality of transducers, eachtransducer producing displacement of the sensor relative to thespacecraft that is orthogonal relative to the displacement produced byeach other transducer;

[0015] servo-control means are also provided for each transducer,suitable for compensating disturbing movements affecting the position ofeach sensor relative to an object to be observed;

[0016] the active material is mounted directly on a matrix ofobservation sensors to form an integrated structure; and

[0017] each observation sensor is a photosensitive charge transferelement.

[0018] In another aspect, the invention provides a device for causing acamera device to be displaced in controlled manner in at least onedegree of freedom relative to the spacecraft. It further comprisesdisplacement means comprising a transducer of an active materialsuitable for deforming under the effect of an electric field and/or amagnetic field. Such a device makes it possible in particular tocompensate smearing associated with the movements of a spacecraftrelative to an object that is remote from the craft, and in particularmovements corresponding to displacements along a trajectory.

[0019] Such a device, coupled with a control system that generates avoluntary displacement relationship depending on the behavior of theplatform on which the camera device is mounted, together with a cameradevice of the desired type, makes it possible at negligible extra costand weight to provide a function serving to obtain maximum resolutionand scanning width for an optical system carried by a spacecraft,without giving rise to energy losses.

[0020] In yet another aspect, the invention provides a spacecraftincluding a camera device as specified above.

[0021] In yet another aspect, the invention provides a method of imagingin space, the method including an operation consisting in displacing acamera device relative to a spacecraft, the method being characterizedin that this operation is implemented by applying an electric fieldand/or a magnetic field to deform an active material interposed betweenthe camera device and the spacecraft, and in that said transducer isdriven in such a manner as to displace the observation sensor incompliance with a variable control relationship adapted to compensatingthe movements induced by the flight dynamics of the satellite.

[0022] In advantageous but optional manner, the method of the inventionpresents the following characteristics taken independently orseparately:

[0023] it includes an operation consisting in compensating the smearingassociated with the movements of a spacecraft relative to an object tobe observed that is remote from said spacecraft, and in particular themovements corresponding to the displacement of said spacecraft along atrajectory;

[0024] it includes an operation consisting in using the active materialto cause a matrix of observation sensors extending mainly in a plane totilt relative to the tangent of the trajectory of the spacecraft;

[0025] it includes an operation consisting in taking a first image ofthe object to be observed from a first position on the trajectory, andthen a second image of said object from a position downstream from saidfirst position, in order to form a stereoscopic image of the object;

[0026] it includes an operation consisting in displacing the cameradevice laterally relative to the trajectory of the spacecraft to form astereoscopic image of the object;

[0027] it includes an operation consisting in causing the camera deviceto turn about an axis perpendicular to the trajectory in order to form astereoscopic image of the object to be observed; and

[0028] it includes an operation consisting in taking a plurality ofimages corresponding to the camera device occupying positions that arejuxtaposed beside one another by displacing the camera device betweentaking two images, the device being displaced laterally relative to thetrajectory of the spacecraft so as to form, after processing all of theimages, an image of the object to be observed that corresponds to afield that is wider than that which would be obtained using only one ofsaid images.

[0029] Other aspects, objects, and advantages of the invention arebetter understood on reading the following detailed description. Thepresent invention is also better understood with the help of referencesto the drawings, in which:

[0030]FIG. 1 is a diagrammatic perspective view of a matrix ofphotosensitive charge transfer elements forming part of a camera devicein accordance with the present invention;

[0031]FIG. 2 is a diagrammatic perspective view of an image being takenby means of a matrix of the type shown in FIG. 1;

[0032]FIG. 3 is a diagram of the smearing that can arise while taking animage in the manner shown in FIG. 2;

[0033]FIG. 4 is a diagram of a camera device in accordance with thepresent invention;

[0034]FIG. 5 shows a variant of the camera device in accordance with thepresent invention as shown in FIG. 4;

[0035]FIG. 6 shows yet another variant of the camera device inaccordance with the present invention as shown in FIG. 4 or FIG. 5;

[0036]FIG. 7 is a diagram showing an implementation of the method inaccordance with the present invention;

[0037]FIG. 8 shows a variant implementation of the method in accordancewith the present invention as shown in FIG. 7;

[0038]FIG. 9 shows another variant of the implementations of the methodin accordance with the present invention as shown in FIG. 7 or FIG. 8;

[0039]FIG. 10 shows yet another variant of the implementations of themethod of the present invention as shown in FIG. 7, FIG. 8, and FIG. 9;and

[0040]FIG. 11 is a diagram showing yet another variant of theimplementations of the method in accordance with the present inventionas shown in FIG. 7, FIG. 8, FIG. 9, and FIG. 10.

[0041] An embodiment of the invention is described below in the contextof imaging from a satellite.

[0042] In this example, images are taken by means of a camera devicethat includes a two-dimensional matrix of photosensitive charge transferelements. To achieve high resolution, these photosensitive chargetransfer elements are photodiodes having a size of about 6 μm, forexample.

[0043]FIG. 1 is a diagram of a matrix 1 of sensors 2 made up of suchphotodiodes. By way of example, a matrix 1 may comprise 4000 rows ofsensors 2 by 4000 columns of sensors 2. The photodiodes are associatedwith an optical device for causing an elementary area of resolution onthe ground to coincide with the area of one detector, i.e. with the sizeof a photodiode.

[0044] Thus, for example, for ground resolution R_(s) of meter order,with a sensor of size T of the order of 6 μm, and an altitude A of about600 km, it is necessary to use an optical system whose focal length isgiven by: $F = {{\frac{A}{R_{s}}T} = {3.6\quad m}}$

[0045] With focal length being imposed in this manner, it is appropriateto ensure that the diffraction spot associated with the optical apertureof the system is of the same order of magnitude as the size of adetector. Which implies, for a wavelength λ, that:$\frac{D}{F}\quad {is}\quad {about}\quad \frac{\lambda}{T}$

[0046] where D corresponds to the diameter of the entrance pupil.

[0047] Thus, with a sensor of size T lying in the range 5 μm to 10 μm,an altitude A of 600 km, and a wavelength of about 0.6 μm, the pupil hasa diameter of about 30 centimeters (cm).

[0048] It is assumed below that the satellite is moving along atrajectory V, with the tangent to the trajectory V defining a directionX. The direction Y is defined as being perpendicular to the direction Xin a plane perpendicular to the optical axis of the camera device. Adirection Z is also defined which is perpendicular to the directions Xand Y.

[0049] As shown in FIG. 3, when the satellite moves during exposuretime, if the matrix 1 follows the satellite, then the image 3 of anobject on the ground moves on the matrix 1 (by way of illustration ofFIG. 3, it moves from the position drawn in continuous lines to theposition drawn in dashed lines). This is what causes smearing. To ensurethat the image is not affected by such smearing that destroys thedesired resolution, each sensor 2 must move in the opposite direction tothe displacement of the satellite along its trajectory, and at the samespeed, throughout the duration of the exposure time. Thus, a satellitetraveling relative to the ground at a speed of 8000 km/s during anexposure time of 500 μs (time corresponding to a signal-to-noise ratioof 40 dB) travels through 4 m relative to the ground, which correspondsto 4 pixels, each pixel corresponding to one sensor 2. In order toconserve resolution of meter order, it is therefore appropriate to movethe camera device through 4 pixels in the opposite direction to thesatellite displacement direction.

[0050] Indeed, to obtain a better signal-to-noise ratio, the exposuretime ought to be 1 ms. To compensate for smearing during such anexposure time, displacement should be obtained through 8 pixels. Thetransducer must therefore provide displacement of 48 μm in 1 ms. Suchdisplacement is entirely compatible with the voltage/displacementcharacteristics of a piezoelectric or magnetostrictive transducer.

[0051] It is even possible to envisage compensating smearing over 2 ms,i.e. 16 pixels, which for an optical system having a diameter of 30 cmwould make it possible to achieve signal-to-noise ratios of better than60 dB (which was previously inconceivable for such small opticalsystems).

[0052] The rigidity of a piezoelectric material, associated with thefineness of control that can be obtained over displacement with thistype of material mean that it is an ideal candidate for performing thisfunction of compensating smearing.

[0053] Such transducers can be constituted, for example, by lithiumniobiate capsules sold by SPK Electronics Co. Ltd., having apiezoelectric constant D equal to or greater than 1000×10⁻¹² meters pervolt (m/V).

[0054] Such transducers are not only advantageous for compensatingsmearing due to the satellite traveling along its trajectory whileexposure is taking place, but also for compensatinq any residualdisturbances during exposure. Under such circumstances, the transduceris advantageously included in a control loop. Until now, thestabilization devices used for compensating residual disturbances havebeen more expensive and heavier than the solution of the invention.

[0055] As shown in FIG. 4, the matrix 1 is associated with twotransducer elements 4 and 6 that are controlled independently of eachother. Each of these transducers 4 or 6 is disposed on one side of therectangular parallelepiped constituted by the matrix 1. To simplify theexplanation below, the sides of this rectangle are assumed to beparallel to the directions X and Y. Naturally, other orientations arealso possible. The two sides each provided with a transducer 4 or 6 areadjacent.

[0056] Advantageously, the transducers and the matrix form an integratedstructure.

[0057] The use of two transducers 4 and 6 generating independentorthogonal displacements makes it possible to implement movementssatisfying any arbitrary displacement relationship. This enablessoftware to handle relationships for controlling lateral displacementrelative to the trajectory V, displacements in forward and reversedirections, along the trajectory V, or in situations that are complexsuch as those associated with rotation of the earth.

[0058] A driving microprocessor 8 calculates the corrections that needto be made in real time. The deformations of the transducers 4, 6 can becomplex, given the mechanical hysteresis to which piezoelectrictransducers are subject, for example. However, since these deformationscan be modelled, they can also be taken into account in the calculationsperformed by the microprocessor 8. The results of these calculations areapplied to analog-to-digital converters 10 and 12. The analog signaloutput by each analog-to-digital converter 10, 12 is applied to anamplifier 14, 16 which in turns feeds the piezoelectric transducers 4,6.

[0059] The transducers 4, 6 are controlled using voltage levels that arerelatively low since the displacements generated by the transducers 4and 6 are of small amplitude. For example, with capsules of the typementioned above, having a dielectric constant D equal to about1000×10⁻¹² m/V, and a thickness of 2 mm, applying a voltage of 18kilovolts (kV) enables displacement to be performed through 90 μm.

[0060] Such displacement corresponds to about 10 pixels.

[0061] It is also advantageous to use a stack of piezoelectric materiallayers, with electrodes in parallel. This reduces the voltage that needsto be applied to obtain the desired displacement.

[0062] For example, with nine stacked layers, it is possible to reducethe voltage that is applied to each layer to 2 kV. This voltage is thenentirely compatible with common high voltage devices. Advantageously, itis also possible to use a gastight housing filled with an inert gas inorder to avoid electric arcing.

[0063] As shown in FIG. 5, in an advantageous variant of the cameradevice of the invention as shown in FIG. 4, the matrix 1 is providedwith a transducer 4, 5, 6, or 7 on each of its sides parallel to thedirections X and Y. In this variant, the matrix is moved in push-pullmode. The piezoelectric transducers 4, 5 and 6, 7 situated on oppositesides of the matrix 1 perpendicular to one of the directions X or Y arecontrolled using opposite voltages.

[0064] As shown in FIG. 6, in yet another advantageous variant of thecamera devices of the invention shown in FIGS. 4 and 5, the matrix 1 isprovided with transducers 4, 5, 6, and 7 on each of its edges parallelto the directions X and Y in order to control displacements along thosedirections. However the matrix is also provided with transducers 8 and 9arranged to drive displacement in the direction Z. If a plurality ofindependent transducers 8 and 9 all arranged to drive displacement inthe direction Z are fixed to different points of the matrix 1, it isthen possible to cause the matrix 1 to tilt relative to a planecontaining axes parallel to the directions X and Y. In other words, suchtilting is an operation which consists in causing the matrix 1 to beinclined relative to the tangent of the trajectory V of the satellite.

[0065] As shown in FIG. 7, this tilting makes it possible to take afirst image of the object to be observed from a first position along thetrajectory of the satellite (continuous lines in FIG. 7), followed by asecond image of the same object from a position downstream from thefirst position (dashed lines in FIG. 7), thereby enabling a stereoscopicimage to be formed of said object.

[0066] As shown in FIG. 8, this tilting also makes it possible to takean image that is smaller but that uses more sensors 2. This operationmakes it possible to further increase resolution in the image. Thedistortion in the image taken in this way due to the matrix 1 beingtilted relative to the optical axis of the camera device is easilycorrected by processing the recorded image. This processing mayoptionally take place on the ground. This titling of the matrix 1relative to the tangent of the trajectory of the satellite mayadvantageously be combined with fine displacement for compensatingsmearing.

[0067] As shown in FIG. 9, displacement in directions parallel to theaxes X and Y makes it possible between taking two images of the objectunder observation to move the camera device laterally relative to thetrajectory of the craft so as to form a stereoscopic image of theobject.

[0068] As shown in FIG. 10, the displacement along directions parallelto the axes X and Y makes it possible between taking two images of theobject to be observed to move the camera device so that it turns aboutan axis perpendicular to the trajectory, so as to form a stereoscopicimage of the object to be observed.

[0069] As shown in FIG. 11, it is also possible by the method of theinvention to take a plurality of images corresponding to positions ofthe camera device that are juxtaposed relative to one another. Thecamera device is thus moved laterally relative to the trajectory of thesatellite between taking two images so as to form, after all of theimages have been processed, an image of the object to be observedcorresponding to a field that is wider than that which can be obtainedusing a single image. It is thus possible to implement an imaging methodthat was previously unknown, making it possible to achieve meterresolution over fields of width that are multiples of the dimension T ofa sensor 2.

[0070] For example, a matrix of 4000 sensors by 4000 sensors can cover aground area of 4 km by 4 km. To cover an area corresponding to a squarehaving a side of 0.40 km, it would be necessary to have a matrix of40,000 sensors by 40,000 sensors. Another solution consists in moving amatrix 1 that comprises a smaller number of sensors 2.

[0071] Thus, if the satellite advances through 4 km relative to theground (4000 pixels) in 4.8 ms, it is possible to compensate forsmearing while taking nine images with exposure times of 500 μs. Thismakes it possible to obtain a swath width, i.e. the distance scanned bythe matrix laterally relative to the trajectory, that is equal to 36 kmusing a matrix 1 of 4000 sensors 2.

[0072] The invention is described above with reference to an example ofa camera device in which images are taken using a matrix ofphotosensitive charge transfer elements. However sensors other thanphotosensitive charge transfer elements can be used when implementingthe invention. It is equally possible to use sensors having a longertime constant, such as sensors in the medium or far infrared range orbolometer type heat detectors. Until now, such sensors have beenincompatible, even at modest resolution, with the sampling frequencyimposed by the displacement of satellites. By means of the invention, itis possible to obtain resolutions that have previously beeninaccessible. Thus, for example, it is possible to detect forest fireswith precision of 5 m to 10 m in the far infrared.

[0073] Advantageously, the invention also makes it possible to usesensors that are of low cost and of standard quality, while neverthelessobtaining very high level radiometric quality, for example with asignal-to-noise ratio better than 40 dB.

[0074] The invention is described above with reference to an embodimentof a camera device mounted on a satellite, however it is evident thatthe invention can be implemented in other applications when it isdesired to image an object that is remote from the camera device, andthe displacements of the camera device are of relatively smallamplitude. In particular, it is possible to use spacecraft of typesother than satellites.

1/ A camera device for mounting on board a spacecraft, the camera devicehaving at least one observation sensor and displacement means (4, 5, 6,7, 8, 9) for controlled displacement of said device relative to thespacecraft with at least one degree of freedom, the device beingcharacterized in that said displacement means comprise at least onetransducer of an active material suitable for deforming dynamicallyunder the effect of a variable electric and/or magnetic field, togetherwith driver means suitable for controlling the transducer so as todisplace the observation sensor in compliance with a variable controlrelationship adapted to compensating movements induced by the flightdynamics of the satellite. 2/ A device according to claim 1,characterized by the fact that the active material is a piezoelectricmaterial. 3/ A device according to claim 1, characterized by the factthat the active material is a magnetostrictive material. 4/ A deviceaccording to any preceding claim, characterized by the fact that thedisplacement means (4, 5, 6, 7, 8, 9) comprise a plurality oftransducers, each transducer producing displacement of the sensor (2)relative to the spacecraft that is orthogonal relative to thedisplacement produced by each other transducer. 5/ A device according toany preceding claim, characterized by the fact that it further comprisesservo-control means for each transducer, suitable for compensatingdisturbing movements affecting the position of each sensor (2) relativeto an object to be observed. 6/ A device according to any precedingclaim, characterized by the fact that the active material is mounteddirectly on a matrix (1) of observation sensors (2) to form anintegrated structure. 7/ A device according to any preceding claim,characterized by the fact that each observation sensor (2) is aphotosensitive charge transfer element. 8/ A device for moving a cameradevice in controller manner in at least one degree of freedom relativeto the spacecraft, the device being characterized by the fact that itcomprises displacement means (4, 5, 6, 7, 8, 9) comprising a transducerof an active material suitable for deforming under the effect of anelectric field and/or a magnetic field. 9/ A spacecraft including acamera device with at least one observation sensor (2) and displacementmeans (4, 5, 6, 7, 8, 9) for controlled displacement of the cameradevice relative to the spacecraft in at least one degree of freedom, thespacecraft being characterized by the fact that said displacement means(4, 5, 6, 7, 8, 9) comprise a transducer of an active material suitablefor deforming under the effect of an electric field and/or a magneticfield, together with driver means suitable for controlling thetransducer so as to displace the observation sensor in compliance with avariable control relationship adapted to compensating the movementsinduced by the flight dynamics of the satellite. 10/ A method of imagingin space, the method including an operation consisting in displacing acamera device relative to a spacecraft, the method being characterizedin that this operation is implemented by applying an electric fieldand/or a magnetic field to deform an active material interposed betweenthe camera device and the spacecraft, and in that said transducer isdriven in such a manner as to displace the observation sensor incompliance with a variable control relationship adapted to compensatingthe movements induced by the flight dynamics of the satellite. 11/ Amethod according to claim 10, characterized by the fact that it includesan operation consisting in compensating the smearing associated with themovements of a spacecraft relative to an object to be observed that isremote from said spacecraft, and in particular the movementscorresponding to the displacement of said spacecraft along a trajectory(V). 12/ A method according to claim 10 or claim 11, characterized bythe fact that it includes an operation consisting in using the activematerial to cause a matrix (1) of observation sensors (2) extendingmainly in a plane to tilt relative to the tangent of the trajectory (V)of the spacecraft. 13/ A method according to claim 12, characterized bythe fact that it includes an operation consisting in taking a firstimage of the object to be observed from a first position on thetrajectory (V), and then a second image of said object from a positiondownstream from said first position, in order to form a stereoscopicimage of the object. 14/ A method according to any one of claims 10 to13, characterized by the fact that, between taking two images of theobject to be observed, it includes an operation consisting in displacingthe camera device laterally relative to the trajectory (V) of thespacecraft to form a stereoscopic image of the object. 15/ A methodaccording to any one of claims 10 to 14, characterized by the fact thatit includes an operation consisting in causing the camera device to turnabout an axis perpendicular to the trajectory (V) in order to form astereoscopic image of the object to be observed. 16/ A method accordingto any one of claims 10 to 15, characterized by the fact that itincludes an operation consisting in taking a plurality of imagescorresponding to the camera device occupying positions that arejuxtaposed beside one another by displacing the camera device betweentaking two images, the device being displaced laterally relative to thetrajectory of the spacecraft so as to form, after processing all of theimages, an image of the object to be observed that corresponds to afield that is wider than that which would be obtained using only one ofsaid images.